OMC to Mercruiser!

Dquinn

Cadet
Joined
Feb 13, 2021
Messages
20
Will an alpha one mercruiser off of a 3.0 bolt directly to a 5.7? Tried finding a 4.3 or 5.7 parts boat but so far 3.0's are everywhere. I'm aware of upper gear ratios in the foot but not sure about bellhousing, power steering etc. I have an 84 harris deck boat with the old omc cobra outdrive. Old style with the rubber boot around it mounted in the transom. Going to redo floors stringers and transom and want to cut hole for alpha and convert to that system. Thanks in advance!
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,159
Will an alpha one mercruiser off of a 3.0 bolt directly to a 5.7? Tried finding a 4.3 or 5.7 parts boat but so far 3.0's are everywhere. I'm aware of upper gear ratios in the foot but not sure about bellhousing, power steering etc. I have an 84 harris deck boat with the old omc cobra outdrive. Old style with the rubber boot around it mounted in the transom. Going to redo floors stringers and transom and want to cut hole for alpha and convert to that system. Thanks in advance!
Ayuh,..... Welcome Aboard,..... You've got a Stringer drive,.... Cobra came out in '86,.....

Keep lookin' for the driveline ya want,....
Piece mealin' a driveline together is extremely expensive, 'n ya run into problems with different vintages,.....
 

Dquinn

Cadet
Joined
Feb 13, 2021
Messages
20
Ayuh,..... Welcome Aboard,..... You've got a Stringer drive,.... Cobra came out in '86,.....

Keep lookin' for the driveline ya want,....
Piece mealin' a driveline together is extremely expensive, 'n ya run into problems with different vintages,.....
Thank you sir. Oh ok thought I had seen cobra on it somewhere that's good to know. Yea ultimate goal is to find a 5.7 boat with power steering and a complete alpha drive with a bad motor or broke hull (whatever would make it a project boat as long as the drive is good) and when it comes time to mount the outdrive just swap the entire thing from the old boat to my boat. All the way to the block if necessary. I've seen lots of pieces of alpha one's for sale but looks like a nightmare trying to get all the right stuff as you said.
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,870
The stringer uses a larger transom hole than the cobra and alpha drives, you will have to fill that in so you have something to mount it to.
 

Dquinn

Cadet
Joined
Feb 13, 2021
Messages
20
The stringer uses a larger transom hole than the cobra and alpha drives, you will have to fill that in so you have something to mount it to.

The stringer uses a larger transom hole than the cobra and alpha drives, you will have to fill that in so you have something to mount it to.
Thanks scott06! Plan at the moment is to totally tear the transom out once the drive is out, rebuild it with plywood possibly coosa board, the fiberglass the out side to build it up to the factory transom and cut the alpha drive hole.
 

Dquinn

Cadet
Joined
Feb 13, 2021
Messages
20
My biggest concern is with the parts boat. I know a 5.7 and 4.3 are comparable but I've been finding a lot of 3.0 boats with the donor alpha drive and I'm curious if anyone has made that engine swap or knows if the shaft for the drive off a 3.0 will go on a 5.7. Have novice experience with boats but have built/tuned motors a few time so I'm confident in mechanical skill just lacking the knowledge of the mating of the drive to different engines.
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,870
My biggest concern is with the parts boat. I know a 5.7 and 4.3 are comparable but I've been finding a lot of 3.0 boats with the donor alpha drive and I'm curious if anyone has made that engine swap or knows if the shaft for the drive off a 3.0 will go on a 5.7. Have novice experience with boats but have built/tuned motors a few time so I'm confident in mechanical skill just lacking the knowledge of the mating of the drive to different engines.
You can slap a 3.0 drive on a 5.7 but tge drive ratio is way different. The flywheel cover is tge same 3.0, 4.3, and 5.7 .

issue bondo was pointing out if you get a 3.0 donor and try to piece a 5.7 in front of it you will be missing a lot of parts like exhust manifolds/wye pipe/elbows etc, wiring harness , cooling hoses, thermostat housing, acessoris and belt drive up front, distributor... the parts cost will kill you.

i swapped a 4.3 for a 5.0, that wasnt bad. I bought a partial Engine package from Michigan motors included the long block intake distributor, exhaust carburetor. I swapped the wiring harness flywheel cover, coupler, other accessories upfront power steering etc. of the 4.3 and everything fit the 5.0 because the 4.3 is a SBC missing two cylinders. I had to buy 5.0 ignition module some cooling hoses throttle bracket and about $30 worth of bolts.

at the very least hold out for a 4.3 donor as that will be a much easier swapped than a 3.0 Donor in terms of having the parts u need.
 

Dquinn

Cadet
Joined
Feb 13, 2021
Messages
20
You can slap a 3.0 drive on a 5.7 but tge drive ratio is way different. The flywheel cover is tge same 3.0, 4.3, and 5.7 .

issue bondo was pointing out if you get a 3.0 donor and try to piece a 5.7 in front of it you will be missing a lot of parts like exhust manifolds/wye pipe/elbows etc, wiring harness , cooling hoses, thermostat housing, acessoris and belt drive up front, distributor... the parts cost will kill you.

i swapped a 4.3 for a 5.0, that wasnt bad. I bought a partial Engine package from Michigan motors included the long block intake distributor, exhaust carburetor. I swapped the wiring harness flywheel cover, coupler, other accessories upfront power steering etc. of the 4.3 and everything fit the 5.0 because the 4.3 is a SBC missing two cylinders. I had to buy 5.0 ignition module some cooling hoses throttle bracket and about $30 worth of bolts.

at the very least hold out for a 4.3 donor as that will be a much easier swapped than a 3.0 Donor in terms of having the parts u need.
Scott06 dumb question maybe but I already have a good running 5.7 in the boat. Are the parts you listed like manifolds and water pump and belt drive and accessories, etc mercruiser specific? Manifolds look super similar to my buddies with mercruiser and was hoping to just swap the bellhousing/flywheel cover back, plumb water to the pump, and might just go thru hull. I'd love to find a 5.7 donor boat! Haha I've found a lot of them but they are mercruiser II so pre alpha one
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,870
Scott06 dumb question maybe but I already have a good running 5.7 in the boat. Are the parts you listed like manifolds and water pump and belt drive and accessories, etc mercruiser specific? Manifolds look super similar to my buddies with mercruiser and was hoping to just swap the bellhousing/flywheel cover back, plumb water to the pump, and might just go thru hull. I'd love to find a 5.7 donor boat! Haha I've found a lot of them but they are mercruiser II so pre alpha one
The only thing youd keep that isnt mercruiser specific is the longblock and intake manifold. All the other components are specific to the obsolete omc stringer set up you want to get rid of.
 

matt167

Rear Admiral
Joined
Sep 27, 2012
Messages
4,187
Your marine 350 if it runs good can be worth hundreds to a person in need of a good 350. If it runs good in a boat, it'll run great in a car.

You could try to find a cracked block 5.0L or 5.7L boat. That will give the correct drive ratio and you could use your good long block. However, it would probably also have cracked manifolds and risers so it would be a slippery slope into $$$$.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,159
Thanks scott06! Plan at the moment is to totally tear the transom out once the drive is out, rebuild it with plywood possibly coosa board, the fiberglass the out side to build it up to the factory transom and cut the alpha drive hole.
Ayuh,...... As someone that's done this quite a few times(restorin' classic hulls, that had stringer power), hold out til ya find the driveline ya want,....
The Alpha, Gen.II has been 'round since 1990, so rotten hulls are 'round with 'em, 'n can be had for under $3500.....
In good runnin' condition, with rotten transoms,....

Once ya get the donor hull, strip out the driveline, 'n cut a big plug outa the transom glass, with the Merc cut-out intact,.....
Determine where, exactly ya want the X dimension, 'n attach the plug outa the donor hull to yer hull, with the crankshaft centerline where it's gotta go,.....
Then from inside the hull, mark out the big OMC hole onto the plug outa the donor hull, pull it off the good hull, cut it outa the plug, 'n use it for the outside skin, glassin' it into the good hull,.....
Now you'll have the cut-out holes where they gotta be, 'n use 'em as a pattern to carve out the new plywood for yer new Merc transom,.....

As noted above, sell off the OMC stuff ya have to somebody still tryin' to get one more lap, outa their OMC,.....
Use that money to buy the driveline ya really want,..... ;)
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
50,530
me personally, like Bondo said.....

sell everything you have from controls/gauges to the prop - yes, I did say controls an gauges to the prop.......

then buy everything you need from controls/gauges to the prop from a donor boat

or simply take the $3k you are going to dump into your hull, and the $3k for the new motor and drive and sell your stuff for about $1k (in pieces as spare parts)....... get a few extra $ and get something at least 20 years newer
 

Dquinn

Cadet
Joined
Feb 13, 2021
Messages
20
me personally, like Bondo said.....

sell everything you have from controls/gauges to the prop - yes, I did say controls an gauges to the prop.......

then buy everything you need from controls/gauges to the prop from a donor boat

or simply take the $3k you are going to dump into your hull, and the $3k for the new motor and drive and sell your stuff for about $1k (in pieces as spare parts)....... get a few extra $ and get something at least 20 years newer
Yea bondo that's a great idea of using the donor boat outer hull to patch in and also have a template. I'm pretty nifty with a tape measure and was just gonna engineer the **** out of it but I like that idea.
And Scott... That has crossed my mind several times haha. And isn't completely out of the question, but I do love that boat! Decisions decisions
 

Dquinn

Cadet
Joined
Feb 13, 2021
Messages
20
Yea bondo that's a great idea of using the donor boat outer hull to patch in and also have a template. I'm pretty nifty with a tape measure and was just gonna engineer the **** out of it but I like that idea.
And Scott... That has crossed my mind several times haha. And isn't completely out of the question, but I do love that boat! Decisions decisions
Which I do realise it will still take some good measuring but still.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
50,530
I loved my first few boats........however I never loved one enough to want to go thru the work to restore the hull and swap drivelines and fill the giant hole from an OMC
 

tank1949

Lieutenant Commander
Joined
Apr 4, 2013
Messages
1,911
Will an alpha one mercruiser off of a 3.0 bolt directly to a 5.7? Tried finding a 4.3 or 5.7 parts boat but so far 3.0's are everywhere. I'm aware of upper gear ratios in the foot but not sure about bellhousing, power steering etc. I have an 84 harris deck boat with the old omc cobra outdrive. Old style with the rubber boot around it mounted in the transom. Going to redo floors stringers and transom and want to cut hole for alpha and convert to that system. Thanks in advance!
I suspect you will have to modify inner mounts/stringer attachments and redesign transom hole. That old of a boat, I also suspect ROT!
 
Top